OBSERVING THE PLANETS - Part 1
Because of its prominence of the planet Saturn in our skies this year - and every year - the number of inquiries regarding what can (or should) be seen on the ringed planet are many.
Saturn is a favorite: majestically presenting its rings (or "ears" as Galileo first referred to them) toward us at a sharp 26-degree angle (the southern hemisphere of the planet is tilted our way right now). The colors of the planet are splendid, with vivid yellows, oranges and whites contrasting nicely in both the rings and the planet itself.
Being a gaseous giant planet like Jupiter, there are NO surface markings to be visible, only faint "belts" which are downward convection currents. Like canyons in the clouds, sunlight cannot reflect from these areas and hence, they appear darker to us in our telescopes than do the surrounding high clouds, or what we call "zones" on the gas planets.
Occasionally, but not as often as on Jupiter, a bright white cloud will be seen on Saturn, these usually forming just north of the North Equatorial Belt (NEB) or immediately south of the South Equatorial Belt (SEB - shown in the attached photo and clearly visible in your telescope). These white clouds are cyclonic storms which force cloudtops high into the Saturnian atmosphere and, hence increase their bright reflectivity.
Commonly, telescope users hear us refer to features on Saturn such as "Cassini's Division," the "Crepe Ring", "Encke's Division," and so on. Sometimes in our haste to explain what an observer SHOULD be able to see in a particular telescope, we forget that the jargon may not be familiar to all telescope users.
Therefore, I have compiled a description of Saturn and its "components" for viewing and a brief overview of what IS visible in common telescopes and what you might expect to see.
Those of you who have read many of my philosophical wanderings understand that I truly believe that ANY observer's FIRST view of Saturn in his or her own telescope in "real time" is an event never forgotten no matter what age we attain. So, at least in my opinion, it is important to know WHAT we can see and something ABOUT that which we do see.
Attached is a NASA/Hubble photograph which I selected primarily because this reproduction (by the time it reaches YOU) is very close to the way that Saturn should look in a very good 5" telescope; smaller telescopes can reveal just about the same detail, but perhaps with not as good contrast and color.
Keep in mind that the following description details features which can be seen under OPTIMAL viewing conditions: 1) when the air is very steady; 2) when Saturn is high in the sky; 3) when your telescope has cooled down (1-2 hours minimum) and 4) under planetary viewing magnification, or about 50x per inch aperture minimum.
Saturn, as we all know by now from Hubble and the many spacecraft which have penetrated the far solar neighborhood, has many more rings (and features) than we can see here on Earth; that applies to even the largest telescopes available.
With common telescopes under good conditions, THREE (3) rings are clearly visible on Saturn (see photo description): 1) the OUTERMOST RING (ring "A"); the MIDDLE and largest ring (ring "B"); and, 3) the CREPE or innermost (ring "C").
The Crepe Ring is VERY transparent and sometimes is difficult unless very high magnification is used. Look for the Crepe ring easiest where the ring appears to curve the sharpest (the "Ansae" at each side of the rings) or where the "C-ring" passes directly IN FRONT of the globe of Saturn; indeed, where the ring passes in front of the globe, it is actually possible even with a 3-inch scope to see the glove THROUGH the Crepe ring! Try it and impress your friends and neighbors!
The "B-ring" is the brightest and usually appears a whitish yellow color; on nights of extremely steady viewing a 5" or larger telescope can depict faint detail within this ring.
Outermost from the globe of the planet is the "A-ring", not as bright as the "B-ring" but most definitely easy and brighter than the elusive Crepe ring. It is always a dull yellow-orange color compared to "Ring B", and it is within this ring that the VERY difficult ENCKE'S DIVISION is seen on perfect nights.
A fourth, very faint, ring can sometimes be seen at very high power on perfect nights, even with smaller telescopes. However, apertures of greater than 12-14" are recommended and even then unskilled observers normally miss the "D-ring" which is found INSIDE the Crepe Ring and just OUTSIDE the ball of the planet itself.
There are two primary ring divisions that we are interested in common telescopes:CASSINI'S DIVISION; and ENCKE'S DIVISION.
Cassini's Division - this is an easy test for your 3" telescope and should be held steady at magnifications of 100x or greater. A MINIMUM of 40-50x in nearly all telescopes should reveal the dark gap which appears like an inky line separating Rings A and B. On nights when the seeing is particularly troublesome, the image of the faint curved line will sometimes come and go, but do not be discouraged...it is NOT your telescope!
Encke's Division - probably not possible to discern in a 3-inch telescope, but a test for a 5", I have clearly seen Encke's division with my ETX 125 on numerous very steady nights; on nights of average seeing, it evades observation. Likewise, observations with a 40" Boller & Chivens Richey-Cretien Cassegrain failed to disclose this very faint ring gap on one night in a Nebraska corn field, but the next night it was sharply distinct, with three other minor ring gaps visible! Those using 8" telescopes and larger should make this a priority test; look for Encke's in the "A-ring", about one-third of the width of that ring from the OUTSIDE edge. For the purists out there, Encke's division is a gap that is only .35" arc, thereby a real test for amateur telescopes; smaller telescopes, although they cannot resolve to that small a measure, can "see" Encke's through the CONTRAST of the "A-ring" on each side of it!
To prove the aforementioned point, note that CASSINI'S division is clearly visible in telescopes of 3" and above (even a good 2.4" refractor). Yet the ACTUAL width of this object is ONLY 0.6 arc seconds! In arc", the outer edge of "Ring B" is 16.9", while the inner edge of "Ring A" is 17.5" from the outer edge of the planet's globe. So, even though your telescope may not be able to actually "resolve" Cassini's, it "sees" it through the bright contrast between the "A" and "B" rings.
DETAILS ON SATURN'S GLOBE
Here is a drawing (from "A Complete Manual of Amateur Astronomy", P. Clay Sherrod, Prentice-Hall 1982, used by permission) that demonstrates the astronomical nomenclature for all gaseous planets: Jupiter, Saturn, Uranus and Neptune. It is the same for all planets. Note that, ASTRONOMICALLY, south is always shown at the TOP of photos and diagrams. Now, the Hubble photos I have provided and the drawing can be compared; although the electronic file of the photo is certainly not as clear as the original, try to make out which "belts" and "zones" you can make out in the southern hemisphere of Saturn.
I can clearly (from a print of this image) make out (from the equator progressive SOUTH): the equatorial zone, the SEB, the South Tropical Zone (STrZ), the South Temperate Belt (STB), the South Temperate Zone (STZ), and the South South Temperate Belt (SSTB), as well as the South Polar Region (SPR).
This is exactly what you can expect to see on a very steady night - even with a 90mm telescope! If you cannot, here are a couple of hints:
1) AVERTED VISION - this is an astronomical trick, not used too much anymore since most people never "look" through telescopes. When you look DIRECTLY at any bright object, particularly a small one like a planet, you are viewing with the worst part of your eye. In this position you view only with the RODS of your retina which - the older you get - deteriorate with age and use. Try this: with Saturn centered, attempt to move your eye so that you never actually stare RIGHT AT the planet but rather skim past it, or look just to one side. This activates the "CONES" of your retina which are more color and light sensitive. It is amazing what you can see with this "averted vision" than without it! Try it also on double stars and deep sky objects.
2) FILTERS - if you think that the planets look "washed out" and your skies are perfect yet little detail is visible, use a filter. The actual filter used depends on which planet you are observing, but we will concentrate here on Saturn. A Wratten #80A light blue filter is perfect for viewing Saturn, providing optimum contrast between the darker belts and zones. Try a Wratten #12 (yellow) to view the rings and provide the very best viewing of Cassini's and Encke's divisions.
FOR YOUR VIEWING PLEASURE....
A couple of quick recommendations for your viewing pleasure while looking at Saturn (refer to the photograph):
1) Look for the delicate "Crepe Ring" (it is seemingly transparent) as it crosses IN FRONT of the globe; you will clearly see it as a darkened streak across the body of Saturn (clearly visible in the photo);
2) The globe of Saturn casts a magnificent SHADOW onto the rings behind it (see photo), providing one of the most spectacular and believable "3-D" images you can have in a telescope!
Enjoy Saturn, and learn to understand what you see when you gaze 800 million miles away. The thrill of your first look at this ringed marvel will never be equaled.....but I can guarantee you that seeing it EVERY OTHER TIME from now until your last glimpse of days....
....this thrill will last your entire lifetime. Enjoy, it is a gift.
P. Clay Sherrod
Arkansas Sky Observatory
Conway / Petit Jean Mountain
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